1. Field of the Invention
The present invention relates generally to an optical device having an optical waveguide structure, and more particularly to an optical device and a manufacturing method therefor which device is suitably used as an optical multiplexer or an optical demultiplexer in a system adopting wavelength division multiplexing.
2. Description of the Related Art
In recent years, our advanced information society has began processing massive amounts of information. Optical fibers, which have a high transmission capacity, have been employed in communications/transmission networks. The transmission rate of information in optical fiber communications has already reached 2.4 Gb/s or 10 Gb/s. However, a further increase in transmission capacity will be necessary to enable a high quality video communication system that is expected to be put to practical use in the future. For example, a transmission capacity exceeding 1 terabit per second (Tb/s) will likely be necessary in a trunk system.
Wavelength division multiplexing (WDM) is one known technique for increasing transmission capacity in optical fiber communications. In a system adopting WDM, a plurality of optical carriers using different wavelengths are independently modulated to obtain a plurality of optical signals. The plurality of optical signals are wavelength division multiplexed by an optical multiplexer, and the resultant WDM optical signal is transmitted over an optical fiber transmission line. On a receiving side, the received WDM optical signals are separated according to wavelength into individual optical signals, by an optical demultiplexer. The individual optical signals are then demodulated to reproduce the transmitted data. Accordingly, by employing WDM, the transmission capacity of a single optical fiber can be increased in proportion to the number of WDM signal channels.
There are variations in the requirements for WDM systems. For example, the number of WDM channels may vary from several channels to about 100 channels, depending upon the system. Further, there may be a wide variations in wavelength spacing, perhaps from 1 nm or less to tens of nm. In applying WDM to a subscriber system, it is necessary to provide the components at low prices, and this is difficult if the components must be specially made. Accordingly, a WDM filter usable as an optical multiplexer and/or an optical demultiplexer for a variety of WDM systems is a key device.
Aside from communications systems, it has also been proposed to use WDM in the field of measurement, and the WDM filter is an important component also in this field.
FIG. 1 is a plan view of a first conventional WDM filter usable as an optical multiplexer and/or an optical demultiplexer. This WDM filter includes a pair of slab optical waveguides (planar optical waveguides) 2 and 4 and a plurality of optical waveguides (arrayed optical waveguides) 6 for connecting the slab optical waveguides 2 and 4. The arrayed optical waveguides 6 have different optical path lengths. More specifically, the arrayed optical waveguides 6 are formed so that for light having a specific wavelength, there is a phase difference of an integral multiple of 2.pi. between any adjacent waveguides 6.
To demultiplex an optical signal, at least one input optical waveguide 8 is connected to the slab optical waveguide 2 on a side opposite to the side having arrayed optical waveguides 6 connected thereto, and a plurality of output optical waveguides 10 are connected to the slab optical waveguide 4 on a side opposite to the side having arrayed optical waveguides 6 connected thereto. Diffraction occurs in a diffraction grating including the optical waveguides 6, and as the result the input optical waveguide 8 and each output optical waveguide 10 are coupled together by a specific wavelength. Accordingly, when a multiplexed (WDM) optical signal is supplied to the input optical waveguide 8, the optical signals from different wavelength channels are respectively output to the output optical waveguides 10.
In the case of using this WDM filter as an optical multiplexer, optical signals from different wavelength channels are respectively supplied to the optical waveguides 10. The optical signals are then wavelength division multiplexed, and the resultant WDM optical signal is output from the optical waveguide 8.
FIG. 2 is a plan view of a second conventional WDM filter usable as an optical multiplexer and/or an optical demultiplexer. This WDM filter includes a slab optical waveguide 12 having end faces 12A and 12B, with a plurality of first optical waveguides 14 optically connected to the end face 12A, and a plurality of second optical waveguides 16 optically connected to end face 12B. That is, a first end portion of each second optical waveguide 16 is optically connected to end face 12B of the slab optical waveguide 12. A second end portion of each second optical waveguide 16 is directly connected to a reflecting element 18. Each second optical waveguide 16 has a substantially uniform width. To have the optical waveguides 16 and the reflecting elements 18 function substantially as a diffraction grating, the second optical waveguides 16 have different optical path lengths. More specifically, the optical waveguides 16 are formed so that there is a phase difference between adjacent second optical waveguide 16. The phase difference is an integral multiple of 2.pi. between adjacent second optical waveguides 16 for reflected light having a specific wavelength reciprocating in the second optical waveguides 16. In this WDM filter, it is sufficient to provide a single slab optical waveguide 12, so that the size of the WDM filter can be made smaller than that of the WDM filter shown in FIG. 1.
In the case of using this WDM filter as an optical demultiplexer, one of the first optical waveguides 14 is used as an input port for a multiplexed symbol, and the others are used as output ports for demultiplexed signals. Conversely, in the case of using this WDM filter as an optical multiplexer, one of the first optical waveguides 14 is used as an output port for the multiplexed signal, and the others are used as input ports.
The WDM filter shown in FIG. 1 has a problem in that it tends to be large. The large size is due to the fact that the optical waveguides 6 must be long enough to generate the optical path length difference required for a diffraction grating and due to the fact that two slab optical waveguides 2 and 4 are required.
The WDM filter shown in FIG. 2 has a problem in that the manufacturing process for the reflecting elements 18 is complicated. For example, for each reflecting element 18 to provide a diffraction grating, it is necessary to carry out a complicated manufacturing process including plural exposures.
It has been proposed to use a simpler process to obtain each reflecting element 18, which simpler process includes the step of forming an end face on each second optical waveguide 16 perpendicular to the optical path (optical axis) of the second optical waveguide. Then, a reflection film is formed directly on the perpendicular end face. Although an end face can be obtained by etching, the etching step causes a substantial deterioration in the perpendicularity of the end face, particularly at edge portions of the end face. Such a deterioration in perpendicularity is partially due to the fact that, for example, the etching rate for silica glass, which may be used as the material of the second optical waveguides 16, is low.
The WDM filters shown in FIGS. 1 and 2 have a common problem in that they have characteristics which are dependent on temperature. That is, since the temperature coefficient of refractive index (refractive index temperature coefficient) of each optical waveguide 6 or 16 is not zero, multiplexing or demultiplexing on the order of .mu.m is affected by temperature changes. As a result, the usable temperature range at which required characteristics can be obtained is narrow.